The present invention relates to an electric machine,                wherein the electric machine comprises a housing in which a stator is arranged and in which a rotor is rotationally supported about an axis of rotation,        wherein the housing extends from a front end to a rear end when viewed in the direction of the axis of rotation,        wherein on a top face the housing comprises a front air inlet opening in the vicinity of the front end, a rear air inlet opening in the vicinity of the rear end, and an air outlet opening therebetween,        wherein the electric machine comprises a front and a rear air conveying element by means of which air is drawn in at the front and the rear air inlet openings and discharged at the air outlet opening during operation of the electric machine,        wherein an auxiliary cooler is mounted on the top face of the housing, which auxiliary cooler covers the front and the rear air inlet openings and the air outlet opening with side walls and a top cover in a hood-like manner, such that the air discharged at the air outlet opening is fed back to the front and the rear air inlet openings,        wherein the auxiliary cooler comprises a front and a rear auxiliary partition,        wherein the front and the rear auxiliary partitions are arranged between the air outlet opening and the front and the rear air inlet openings and extend upward from the top face of the housing.        
Such electric machines are generally known.
Electric machines having an auxiliary cooler are known in various embodiments. They are on the one hand divided into single-flow and double-flow machines in respect of the path of the primary air which flows through the electric machine. Single-flow electric machines have one air inlet opening in the vicinity of the front end and one air outlet opening in the vicinity of the rear end. Double-flow electric machines have—as explained above—an air inlet opening in the vicinity of each of the front end and of the rear end and an air outlet opening therebetween. Electric machines having an auxiliary cooler are furthermore subdivided in respect of the cooling of the primary air in the auxiliary cooler into electric machines in which the primary air is cooled by water and into electric machines in which the primary air is cooled by secondary air. The present invention relates to electric machines having an auxiliary cooler, in which machines the primary air is cooled by secondary air.
With regard to such electric machines, many tubes running parallel to the axis of rotation are present in the auxiliary cooler. The tubes are open to the outside. The air (primary air) discharged from the electric machine at the air outlet opening circulates around the tubes and as a result the tubes absorb the heat contained in the circulating primary air. This means that the tubes cool the primary air before the primary air is fed back to the electric machine. The secondary air flows through the tubes, thereby cooling the tubes, and the secondary air is thus heated.
The direction of flow of the secondary air is parallel to the axis of rotation, namely from the rear end toward the front end of the electric machine. Due to this circumstance the cooling capacity is greatest at the rear end of the auxiliary cooler, at which the secondary air is introduced into the tubes. The cooling effect at the front end is however smallest. According to the prior art the primary air fed to the front air inlet opening is therefore warmer than the primary air fed to the rear air inlet opening. Temperature differences of approximately 20 Kelvin result in practice.
The temperature differences occurring with regard to the primary air fed to the electric machine by way of the front and rear air inlet openings result in the electric machine being cooled less well in the front region than in the rear region. Due to this circumstance a performance level of the electric machine determined by its construction can often not be fully utilized. Furthermore, this results in a reduced service life for the electric machine. As a rule of thumb, an increase in temperature of 10 Kelvin results in a 50% reduction in the service life of the electric machine.